As for a surface light source, an edge lighting type lighting apparatus is usually used. As illustrated in FIG. 7, the edge lighting type lighting apparatus includes a cathode tube 31, a reflecting section 32, and a photoconductive member 33. The cathode tube 31 is a thin tube type linear light source and driven by a high frequency current generated by a high voltage. The photoconductive member 33 is a light transmitting plate, and converts light emitted from the cathode tube 31 into a surface light source when the cathode tube 31 is disposed adjacent to an edge surface 34a thereof. The reflecting section 32 covers the cathode tube 31 so that the light from the cathode tube 31 is efficiently reflected and guided to the photoconductive member 33.
The photoconductive member 33 has a transparent resin plate 34 with good light transmitting properties, for example, made of an acrylic resin. Irregular reflection surfaces 35 in the form of dots are formed on the rear surface of the transparent resin plate 34 by screen printing, for example. A reflecting plate 36 made of a white polyester film is laminated to cover the irregular reflection surfaces 35. The irregular reflection surfaces 35 and the reflecting plate 36 constitute a reflecting member of the surface light source.
Also formed on the front surface of the transparent resin plate 34 is a diffusion resin sheet 37 for evenly diffusing light which has been reflected by the reflecting plate 36 and passed through the transparent resin plate 34. Such a structure provides the photoconductive member 33 capable of converting a linear light source into a surface light source.
In order to efficiently conducting light emitted from the surface of the cathode tube 31 to an edge surface 34 of the transparent resin plate 34, a reflecting section 32 positioned along the periphery of the cathode tube 31 includes a base material 39 and a reflecting surface 38. The reflecting surface 38 is a metal mirror and formed on the inner surface of the base material 39.
As to the reflecting section 32, in order to achieve efficient reflection, the reflecting surface 38 is formed by uninterruptedly and evenly depositing a metal thin film such as silver and aluminum on the surface of the base material 39 made of, for example, a polyester film by vacuum evaporation.
As to the reflecting surface 38, it is generally known that a metal thin film made of silver reflects light most efficiently, and therefore silver is often used as material for the reflecting surface 38. However, when the reflecting surface 38 is formed by silver, decreasing the thickness of the lighting apparatus is prevented by the following problems.
The cathode tube 31 is turned on by a high frequency current generated by a high voltage. However, when the reflecting surface 38 is formed by silver on the reflecting section 32, if the reflecting section 32 is positioned close to the cathode tube 31 to reduce the thickness of the lighting apparatus, part of a current which should flow to the cathode tube 31 is caused to flow as a leakage current to the reflecting surface 38 due to an electrostatic capacity between the cathode tube 31 and the reflecting surface 38. Consequently, an amount of current which should flow to the cathode tube 31 can not be ensured due to the leakage current, thereby decreasing the brightness of the lighting apparatus.
In order to prevent such a leakage current, the following methods have been used. More specifically, Japanese Publication for Examined Patent Application No. 66519/1992 discloses the formation of a reflecting surface by a white foaming resin surface. Japanese Publication for Unexamined Patent Application No. 134231/1993 discloses the use of a surface of a molded article formed by plastics of high reflectivity by extrusion molding as a reflecting surface.
These methods prevent the leakage current by removing the reflecting surface 38 as a metal conducting member disposed close to the cathode tube 31 and using as an insulating member a foaming resin sheet or a white surface of a plastic molded article having high reflectivity for the reflecting surface.
However, since the reflecting surfaces disclosed in the above-mentioned publications are made of foaming sheets or plastics of high reflectivity, they are inferior to a reflecting surface made of silver in terms of reflecting efficiency.
More specifically, according to the data obtained by the present inventor through comparisons between the reflecting surfaces of the above-mentioned publications and the reflecting surface made of silver, it is confirmed that the reflecting surfaces of the above-mentioned publications lower the surface brightness by about 5 to 20 %.
In short, although the conventional lighting apparatuses prevent a leakage current, they eventually cause a lowering of surface brightness due to the lowered reflecting efficiency.
When a liquid crystal display device incorporates such a lighting apparatus as a backlight, it is difficult to achieve a reduction in the thickness and power consumption and to obtain sufficient brightness.